The typical thyroid epithelial cell can take in colloid from the follicular lumen by macropinocytosis. It can also phagocytose red blood cells. We propose to study the mechanism of these processes by electron microscopy, histochemistry and related techniques. BIBLIOGRAPHIC REFERENCES: Zeligs, J.D. and Wollman, S.H.: Ultrastructure of Blebbing and Phagocytosis of Blebs by Hyperplastic Thyroid Cells in vivo. J. Cell Biol. 72: 584-594, 1977. Zeligs, J.D. and Wollman, S.H.: Ultrastructure of Erythrophagocytosis and Red Blood Cell Fission in Thyroid Epithlelial Cells in vivo. J. Ultrastruc. Res. 59: 59-69, 1977.